1. Technical Field
The present invention relates to precast concrete structural units for use in the construction of retaining walls and particularly to such structural units for use in the construction of sealing load-bearing retaining walls.
2. Background Art
Load bearing retaining walls such as roadway bridge abutments traditionally are constructed of concrete poured at the site. This type of construction is slow and labor intensive, involving setting up and stripping forms.
To save time and expense, systems of precast concrete modules have been developed for use in erecting retaining walls in general, and some of these modular systems have been proposed for bridge abutments. One such modular system is disclosed in U.S. Pat. No. 4,372,091 of BROWN et al. The precast modules of BROWN et al. are bin-type structures having a pair of spaced longitudinally extending side panels and a pair of laterally extending connecting arms that combine to form a rectangular bin open at both top and bottom. The side panels are rectangular and serve as the front and back faces of a retaining wall when the modules are stacked in rows like bricks or building blocks. The upper and lower edges of the connecting arms are formed with mortise and tenon lateral interlocking means that increase the structural integrity of the assembled retaining wall.
The connecting arms of the BROWN et al. modules are spaced from the ends of the side panels by one-quarter of the length of the panels, so that when the modules are placed end-to-end in a row, bins are formed between each module that are equal in size to the bins formed between the pair of connecting arms of the module. The modules of the next superposed row can be stacked either in vertical alignment with corresponding subjacent modules or staggered half-and-half on two adjoining subjacent modules. Either way, this results in interior columnar openings which are continuous vertically and which are adapted for the receipt of fill material. The fill material adds weight to the modules to offset forces applied by additional fill to the rear of the wall. The assembled modular wall thus acts as a "gravity wall."
The thickness of both side panels and the connecting arms of the BROWN et al. modules increases from top to bottom, both to provide draft for ease in removing from the mold and to provide inclined side surfaces so that a major part of the weight of the fill is effectively transferred to become part of the weight of the modules.
The precast bin-type modules of BROWN et al. are effective in constructing a retaining wall which is subjected primarily to lateral forces by the fill behind the wall. The patent also suggests that the modules can be used to construct a bridge abutment, which must carry substantial vertical loads as well as resist lateral horizontal forces. The design of the module is not well adapted to this function, however, because the columns of fill inside the bins will tend to settle over time, even if compacted. Consequently, the vertical live and dead loads of the bridge must be carried essentially entirely by the precast concrete module structure. To distribute these loads uniformly to both side panels and both connecting arms of each module in the top row, BROWN et al. provide a concrete pad that overlies the entire module. The ends of the bridge girders rest on a bearing member that is located back from the face of the abutment by a distance at least half the width of the upper row of modules to properly distribute the load through the composite wall of modules down to a footing. This arrangement can increase the distance between supports of the bridge girders by several feet and correspondingly increase the weight and cost of the bridge materials. In addition, the ultimate vertical load-bearing capability of the modules is limited by the thickness of the side panels and connecting arms. For economy, however, this thickness should be no greater than is necessary for the normal usage of the modules in a retaining wall having no substantial vertical loading.
U.S. Pat. Nos. 3,981,038 and 4,564,967 of VIDAL disclose a bridge abutment construction system that uses precast concrete wall panels that are retained by elongated steel strip reinforcing members attached to the rear of each panel and extending into a stabilized earth mass. This is not a gravity wall system; the wall panels resist horizontal loading forces through offsetting frictional forces between the stabilized earth mass and the steel strip reinforcing members. In U.S. Pat. No. 3,981,038, the stabilized earth mass behind the wall supports the vertical load exerted by the ends of the bridge girders. In U.S. Pat. No. 4,564,967, VIDAL presents an improvement in which the vertical load of the deck of the bridge is supported independently of the earth mass, which absorbs any horizontal forces. The vertical support means comprises a plurality of vertical concrete pillars or columns connected to the rear faces of the panels. The forms for the columns comprise pipe sections either formed integrally with the panels or attached to the rear faces of the panels. The abutment is built up of successive rows of panels, with reinforcing strips for each row of panels laid out on successive layers of compacted earth. The panels are vertically aligned so that the pipe sections cooperate to form a vertical form extending from a footing to the top of the wall. Concrete is then poured into the vertical pipe forms, and a beam seat is mounted on the pillars.
These support columns of VIDAL '967 are relatively thin and require the support of the stabilized earth around each column and attachment to the face panels to prevent buckling.
Since the panels must be vertically aligned in order to align the pipe sections, the resulting wall has a series of vertical joints, making it inherently less stable than a wall having staggered rows of panels. In addition, because of the large volume of stabilized earth required behind the abutment wall, the VIDAL system is suitable primarily for bridges having filled approaches. Even in such applications, the attachment and careful placement of the elongated reinforcing members between layers of stabilized earth is labor intensive and time consuming compared with the rapid setup of the bin-type modules of BROWN et al.
A further disadvantage of prior retaining walls and bridge abutments of the modular type is that moisture in the fill behind the wall will seep through the joints between the face panels of the modules. This is unsightly and eventually can lead to erosion and corrosion of the concrete facing.